The present invention relates to a light beam scanning system such as a laser beam scanner for use in a laser beam printer or tile like, and more particularly to a photodetector for detecting a scanning beam in such a light beam scanning system.
Light beam scanning systems such as a laser beam scanner for use in a laser beam printer or the like include a scanning element such as a polygonal mirror for scanning a laser beam emitted from a laser beam source. The laser beam is modulated by information that is to be printed. The laser beam, after being scanned and reflected by the polygonal mirror, is applied to a photosensitive medium such as a photosensitive drum to write the information on the photosensitive drum. The output level of the laser beam is monitored, and fed back to the laser beam source, which controls the laser to maintain a constant laser beam output level.
The light beam scanning systems also have a photodetector for detecting the scanning laser beam at a starting end of the stroke of each of scanning cycle. Each time the scanning laser beam is applied to the photodetector, the photodetector produces an output signal, and a processing circuit connected to the photodetector compares the output signal with a threshold level, thereby generating a rectangular horizontal synchronizing signal. The scanning laser beam is controlled to start writing the information on time photosensitive drum based on the horizontal synchronizing signal.
In the conventional light beam scanning systems, the photodetector is directed such that the scanning laser beam falls perpendicularly to the photosensitive surface of the photodetector.
One problem with the conventional orientation of the photodetector is that since the scanning laser beam is applied perpendicularly to the photosensitive surface of time photodetector, the reflection of the scanning laser beam from the photosensitive surface returns to the laser beam source, tending to impair the adjustment of the output level of the laser beam emitted from the laser beam source.
According to another drawback, the laser beam reflection may possibly be diffused by the photosensitive surface of the photodetector. When the diffused laser beam reaches the photosensitive drum, it may develop a ghost in the electrostatic latent image formed on the photosensitive drum.
The photosensitive surface of the photodetector has a transparent insulating film and a slit film thereon. If the photodetector is installed with an error, i.e., if the photosensitive surface is oriented improperly, then the transparent insulating film or the slit film reflects and diffuses the scanning laser beam. When the scanning laser beam is thus reflected and diffused, the photodetector generates an output signal which contains a waveform distortion N as shown in FIG. 1 of the accompanying drawings. The signal distortion N is liable to adversely affect the detection of the timing to start writing the information on the photosensitive drum.